1. Field of the Invention
The present invention relates to an inverter apparatus for supplying a power to a load the inverter apparatus being in interconnection with an AC power system. The present invention also relates to an inverter apparatus islanding operation detecting method for detecting the event that the inverter apparatus has started an islanding operation as dissociated from the AC power system due to a power failure or the like of the AC power system.
2. Description of the Background Art
Conventionally, there has been an inverter apparatus as shown in FIG. 14. This inverter apparatus 100 subjects a DC power inputted from a DC power source 101 to PWM (Pulse Width Modulation) control in an inverter main circuit 102, thereby converting the DC power into an AC power. Then, after a high-frequency component is removed from the converted AC power by a filter comprised of a reactor 103 and a capacitor 104, the resulting AC power is supplied to a load 105. The load 105 is further supplied with an AC power from an AC power system 108 via a breaker 106 and a pole transformer 107, so that the inverter apparatus 100 is operated in interconnection with the AC power system 108.
The inverter main circuit 102 is subjected to the PWM control as follows. That is, a load supply voltage V.sub.o supplied to the load 105 is detected by a voltage detector 109. Then, the detected load supply voltage V.sub.o is inputted to a band-pass filter 110 to pass through it and made to be a sine wave signal Va which is inputted to a current reference generator circuit 111. The current reference generator circuit 111 multiplies the inputted sine wave signal Va by an output command signal Vc so as to generate a current reference signal Ic and input it to an error amplifier circuit 112.
On the other hand, an output current I.sub.o of the inverter main circuit 102 is detected by a current detector 113 and inputted to the error amplifier circuit 112. The error amplifier circuit 112 generates an error amplification signal E based on the inputted current reference signal Ic and the output current I.sub.o and inputs it to a PWM control circuit 114. The PWM control circuit 114 generates a gate pulse signal Gp by executing a pulse width modulation process based on the inputted error amplification signal E and inputs it to a gate drive circuit 115. The gate drive circuit 115 switchingly controls switching elements (not shown) constituting the inverter main circuit 102 by the inputted gate pulse signal Gp, thereby converting the DC power of the DC power source 101 into an AC power.
The inverter apparatus 100 detects the event that the inverter apparatus has entered into an islanding operation state as follows. It is to be noted that the islanding operation means a state in which the DC power source 101 (e.g., a solarlight power generation system) continues supplying a DC power when the AC power system 108 fails.
When the inverter apparatus 100 enters into the islanding operation state, a voltage abnormality and a frequency abnormality occur in the load supply voltage V.sub.o supplied to the load 105. This is for the reasons as follows. That is, the amplitude and phase of the current reference signal Ic relative to the load supply voltage V.sub.o are as shown in FIGS. 15A and 15B due to the transfer characteristics of the band-pass filter 110. Therefore, when the breaker 106 is opened thereby making the inverter apparatus 100 enter into the islanding operation state and when a reactive power supplied from the inverter apparatus 100 and a reactive power required by the load 105 do not coincide with each other, the output of the inverter apparatus 100 (referred to as an inverter output hereinafter) exhibits variations such as (i) deviation of its frequency from the rated frequency (e.g., 50/60 Hz when the AC power system 108 is the commercial power source) f.sub.o, (ii) reduction of its current value, and (iii) reduction of its voltage value. For example, in the case where the load impedance of the inverter apparatus 100 when it enters into the islanding operation state has a delayed phase .phi. as shown in FIG. 15B, the frequency of the inverter output balances when it is increased up to f.sub.1. In this stage, the current value of the inverter output reduces from I.sub.0 to I.sub.1, and the voltage value reduces in accordance with it.
Therefore, this inverter apparatus 100 is provided with a voltage abnormality detector circuit 116 and a frequency abnormality detector circuit 117 for the detection of the voltage abnormality and the frequency abnormality of the load supply voltage V.sub.o and decides whether or not the inverter apparatus 100 has entered into the islanding operation according to the result of detection. Based on the result of decision, the operation of the gate drive circuit 115 is stopped, so that the output of the inverter main circuit 102 is stopped and the DC power source 101 is disconnected from the AC power system 108 by a system linking relay (not shown) of the inverter main circuit 102, thereby preventing the islanding operation of the inverter apparatus 100.
However, the background art inverter apparatus 100 that prevents the islanding operation in the aforementioned manner has had the problem that the frequency variation and the voltage variation of the inverter output reduce when the power factor of the load impedance of the inverter output is proximate to one, causing the islanding operation state to be hardly detected by the voltage abnormality detector circuit 116 or the frequency abnormality detector circuit 117.
For the above reasons, there has been conventionally proposed a method for detecting the islanding operation state by detecting a distortion generated in the output waveform of the inverter output in the islanding operation state. This is as follows. That is, when the inverter apparatus 100 enters into the islanding operation state, an excitation current is supplied from the inverter apparatus 100 to the pole transformer 107, and this excitation current causes a distortion at or around the zero-crossing point of the output waveform of the inverter output supplied to the load 105. Therefore, by measuring the distortion and a rate of change of the distortion, the islanding operation state is detected.
However, in order to detect the islanding operation in a manner as described above, it is required to newly provide a circuit for detecting the distortion and the rate of change of the distortion. This has caused an increase in number of the components and a complication of the construction of the inverter apparatus, which has been a factor for increasing the cost of the inverter apparatus.